Because of their performance and compact size, avalanche photodiodes have long been considered a potential replacement for PMTs in positron emission tomographic instrumentation (PET). The expense of obtaining large area avalanche photodiodes in sufficient quantity, however, has been prohibitive. A new design of avalanche photodiode has been described in the technical literature which promises reduced cost due to its simpler design. This is known as the Avalanching Metal-Resistance-Semiconductor (MRS) detector, which has been invented by a group in the Ukraine. Tests show them to be very sensitive to optical photons. In Phase I, we plan to build on this MRS technology by introducing an altered version of this photodiode design. This new design will utilize avalanche gain in a similar way to the MRS design, but it eliminates the use of the resistive film, which has been shown to be problematic in detector fabrication. A PET instrument based on our new proposed detector design will be suitable for human studies, be high performance, and compact. Most importantly, the proposed detectors will be very 'manufacturable', meaning they will be affordable, have consistent specifications, and be in abundant supply. This Phase I demonstration will be made by fabricating and testing MRS detectors with scintillator crystals. PROPOSED COMMERCIAL APPLICATIONS: The proposed research will be aimed at producing low cost, high performance photodiodes for use in PET scanner equipment. Besides PET scanners, these photodiodes would also be applicable to other medical imaging instrumentation such as gamma cameras and surgical probes. These photodiodes would also find research applications in high energy physics, nuclear physics, and space missions.